Optical resin materials which are characterized by a distributed refractive index have proved useful in the construction of optical conductors such as, optical fibers, optical waveguides, optical integrated circuits, and the corresponding preforms from which these optical conductors are fabricated. In general, plastic or polymeric optical fibers (POF) are considered an attractive alternative to copper cable and glass optical fibers. Typically, the plastic optical fiber (or thin, flexible optical rod) has an elongated core within which the majority of the light travels in a generally axial direction and a sheathing layer which coaxially surrounds the core and confines the light to the core due to its having an index of refraction less than that of the core.
The refractive index distribution of plastic optical fibers can be classified as either a gradient (or graded) index or step index. However, gradient index plastic optical fibers (GI POF) are preferred over step index fibers for many data communication applications due to their superior bandwidth capacity. The index of refraction in a gradient index plastic optical fiber has a distribution that continuously changes within the core of the fiber, generally decreasing radially from a maximum value at the core central axis outwardly until it approaches the lower index of refraction of the sheathing at or near the core-sheathing interface. Due to this continuously varying refractive index within the core, the optical fiber acts like a lens tending to refocus light rays, reducing their propagation in non-axial directions, so that light rays entering the core at a small angle, with respect to the axis, follow undulating paths with relatively small deviations from the axial direction when compared to light propagation in a step index type fiber. In addition, the speed of the light rays following undulating paths is higher in the regions of lower refractive index so that the total travel time for light rays following undulating paths is nearly equal to those following a straight axial path. This results in, for example, a fiber with a wider bandwidth of transmission with minimal modal dispersion and a more rapid information flow than that obtained with step index plastic optical fibers.
In general, typical methods of fabricating gradient index plastic optical fibers involve preparation of a polymeric sheathing and a polymeric core disposed within the sheathing in a coaxial configuration. The refractive index of the core and sheathing are different and, for most optical conducting applications, the refractive index of the core is greater than that of the sheathing. Frequently, the core is made of the same polymer as that which comprises the sheathing but, in addition, further includes a non-polymeric substance (commonly referred to as a dopant) which increases the refractive index of the core so that it is greater than that of the sheathing. (See for example, U.S. Pat. No. 5,541,247 to Koike.)
However, currently available methods of fabrication have significant shortcomings. For example, the type and amount of dopant substances which can be incorporated into the core and still provide a gradient index plastic optical article which maintains both sufficient optical transparency and an acceptable difference in the refractive index between the sheathing and the core, are limited. Therefore, a need exists for methods and materials useful for fabricating improved gradient index plastic optical articles.
One aspect of the present invention is based upon the discovery that a gradient index plastic optical article having excellent optical characteristics can be produced using a method of fabrication that incorporates a low refractive index dopant (i.e. having a refractive index lower than that of the polymer comprising the sheathing but without the dopant) in the sheathing of the article.
The present invention in another aspect relates to a gradient index plastic optical article, and methods of processing the article. The methods of the invention provide for the use of a significantly broader selection of dopant and polymeric materials which can be used to produce a functional gradient index plastic optical article with excellent optical characteristics. For example, the methods of the invention allow for control of the gradient refractive index of the material and for a wider range of differences in refractive indicies between the core and sheathing for a given concentration of core dopant thereby producing a gradient index plastic optical article with a low loss due to light attenuation and broad transmission bandwidth, having a high level of transparency, a substantial absence of bubbles and good environmental stability, for example, enhanced thermal stability and resistance to humidity.
One method for forming a gradient index plastic optical article according to the invention comprises the steps of: (a) forming a transparent tube of sheathing material including at least one sheathing polymer and at least one sheathing dopant; and (b) forming a transparent core within the sheathing tube produced in step (a) by: (i) filling the interior space of the sheathing tube with a core solution including at least one polymerizable core monomer which upon polymerization has a refractive index greater than that of the sheathing tube; and ii) allowing the polymerizable core monomer to polymerize thereby forming a polymeric core having a refractive index greater than that of the sheathing tube such that the article is suitable to conduct light at at least one wavelength with an attenuation less than 500 dB/km. The core solution can include an optional core dopant. When present, the core dopant will have a refractive index greater than that of the polymer obtained upon polymerization of a core monomer solution polymerized under the same conditions but not including the core dopant. The product thus obtained, is a gradient index plastic optical article having an outer sheathing and an inner core both at least partially transparent to light at at least one wavelength. The refractive index of the central axis of the core will be greater than that of the sheathing such that the article is suitable to conduct light at at least one wavelength with an attenuation less than about 500 dB/km, with the refractive index of the core preferably gradually decreasing in a radial direction from the central axis of the core to the periphery of the core at the core-sheathing interface. In general, the article is fabricated in the shape of a preform rod. Preferably, the preform rod has a cylindrical shape which can be drawn into fibers.
In one embodiment, the sheathing tube is made by extrusion methods. Alternatively, the sheathing tube can be produced by: (a) placing into a polymerization container a sheathing solution including at least one sheathing polymerizable monomer and at least one sheathing dopant, the sheathing dopant having a refractive index lower than that of the polymer obtained by the polymerization of a sheathing monomer solution under the same conditions but not including the sheathing dopant; and (b) causing the sheathing monomer of the sheathing solution to polymerize within the polymerization container into a cylindrical sheathing tube at least partially transparent to light at at least one wavelength. The invention further provides a method for forming a gradient index plastic optical fiber. In the method, the gradient index plastic optical article is prepared, for example as described above, in the shape of a preform rod which is then be subjected to hot-drawing at a predetermined temperature and speed suitable to produce a fiber useful as an optical conductor. In one embodiment, the monomer of the sheathing solution and the monomer of the core solution are the same. Suitable monomers include those which form polymers that are substantially amorphous and capable of conducting light at the desired wavelength(s). For embodiments where the core polymer and the sheathing polymer are the same, when a core dopant is used it will be different from the sheathing dopant.
In another aspect gradient index plastic optical articles of the invention comprise: (a) a polymeric sheathing that is at least partially transparent to light at at least one wavelength including at least one sheathing polymer and at least one sheathing dopant, where the sheathing dopant has a refractive index which is less than that of the sheathing polymer; and (b) a polymeric core, coaxially disposed within the sheathing, including at least one core polymer and having a refractive index at the central axis of the core greater than that of the polymeric sheathing. In some embodiments, the polymeric core further includes at least one core dopant, the core dopant, when present, having a refractive index which is greater than that of the core polymer. In preferred embodiments, the core dopant has a concentration gradient in a specific direction.
In some embodiments, the plastic optical article is in the shape of a cylindrical preform rod. In other embodiments, the article is in the shape of a cylindrical fiber having an outer diameter preferably between about 0.1 millimeter and about 1 millimeter.
In yet another aspect, the invention involves a gradient index plastic optical article with a polymeric sheathing and a polymeric core. The polymeric sheathing is at least partially transparent to at least one wavelength of light and includes a sheathing polymer and a sheathing dopant, where the sheathing dopant has a refractive index which is less than that of an equivalent polymeric sheathing without the sheathing dopant. The polymeric core of the article is polymerized within the sheathing, is at least partially transparent to at least one wavelength of light and includes a core polymer. The polymeric core also has a gradient in refractive index in a specific direction that is established by redistribution of a dopant during polymerization of a core solution including a polymerizable core monomer.
In another aspect, the invention provides a method for forming a gradient index plastic optical article. The method involves forming a tube of polymeric sheathing material that is at least partially transparent to at least one wavelength of light from at least one polymerizable sheathing monomer and a sheathing dopant. A polymeric core that is at least partially transparent to at least one wavelength of light is then formed within the tube by filling the tube with a composition including at least one polymerizable core monomer and polymerizing the monomer. The polymeric core thus formed has a gradient in refractive index in a specific direction.
The invention also involves a gradient index plastic optical article which has a polymeric sheathing that includes a sheathing dopant.
In another aspect, the invention involves a gradient index plastic optical article with a polymeric sheathing and a polymeric core. The polymeric sheathing is at least partially transparent to at least one wavelength of light and includes a sheathing polymer. The polymeric core of the article is polymerized within the sheathing, is at least partially transparent to at least one wavelength of light and includes a core polymer and a specific overall concentration of a core dopant that has a refractive index greater than that of the core polymer. Furthermore, the core dopant has a concentration gradient within the core in a specific direction that is established by redistribution of the core dopant during polymerization of a core solution including a polymerizable core monomer. The polymeric sheathing of the article is constructed and arranged so that the difference in refractive indices between the central axis of the polymeric core and the polymeric sheathing exceeds the difference in refractive indices between the central axis of the polymeric core and the sheathing polymer.
In one aspect, the invention involves a gradient index plastic optical article with a polymeric sheathing and a polymeric core. The polymeric sheathing is at least partially transparent to at least one wavelength of light and includes a sheathing polymer. The polymeric core of the article is coaxially disposed within the sheathing, is at least partially transparent to at least one wavelength of light and includes a core polymer and a core dopant that has a refractive index greater than that of the core polymer. The core dopant is present in the polymeric core at a first overall concentration sufficient to create a difference in refractive indices between the central axis of the core and the sheathing of a desired value. In addition, the core dopant has a concentration gradient within the core in a specific direction. The polymeric sheathing of the article is constructed and arranged so that the maximum service temperature of the article exceeds that of an equivalent article except having a sheathing comprised of only sheathing polymer and having a second overall core dopant concentration required to create a difference in refractive indices between the central axis of the core and the sheathing equal to the same desired value. In general, this increase in the permissible service temperature for articles manufactured according to the present invention having a particular difference in refractive indices between core and sheathing is enabled by the ability to use a lower amount of core dopant in order to create the desired difference in refractive indices.
In yet another aspect, the invention involves a gradient index plastic optical article with a polymeric sheathing and a polymeric core. The polymeric sheathing is at least partially transparent to at least one wavelength of light and includes a sheathing polymer. The polymeric core of the article is coaxially disposed within the sheathing, is at least partially transparent to at least one wavelength of light and includes a core polymer and a core dopant that has a refractive index greater than that of the core polymer. The core dopant is present in the polymeric core at a first overall concentration sufficient to create a difference in refractive indices between the central axis of the core and the sheathing of a desired value. Furthermore, the core dopant has a concentration gradient within the core in a specific direction. The polymeric sheathing of the article is constructed and arranged so that at least one wavelength of light is conducted by the article with less attenuation than by an equivalent article except having a sheathing comprised of only sheathing polymer and having a second overall core dopant concentration required to create a difference in refractive indices between the central axis of the core and the sheathing equal to the same desired value.
In one aspect, the invention involves an optical preform article. The preform includes a polymeric sheathing, which is at least partially transparent to at least one wavelength of light and has a refractive index of a first value at that wavelength. The polymeric sheathing includes a sheathing polymer and a plasticizer. The preform also includes a polymeric core, which includes a core polymer, that is polymerized within the sheathing and is at least partially transparent to the same wavelength(s) of light as the polymeric sheathing, and which has a refractive index of a second value at the central axis of the core at that wavelength. The preform is fabricated so that the second value of refractive index (i.e. at the central axis of the polymeric core) exceeds the first value (i.e. of the sheathing).
In another aspect, the invention involves a method for making a plurality of optical preform articles. The method involves forming a plurality of polymeric sheathings, each of which includes a sheathing polymer, is at least partially transparent to at least one wavelength of light, and has a refractive index of a first value at that wavelength. The method also involves forming a plurality of polymeric cores, each of which includes a core polymer, that is coaxially disposed within the sheathing and is at least partially transparent to the same wavelength(s) of light as the polymeric sheathing, and which has a refractive index of a second value at the central axis at that wavelength that exceeds the first value of the sheathing. The region of contact between the sheathings and the cores thus formed defines a plurality of interfaces, with essentially all of the plurality of interfaces being essentially free of visible bubbles. In other words, the invention enables a large number of preforms to be made, each of which is essentially free of visible bubbles along its entire xe2x80x9cas polymerizedxe2x80x9d length (e.g. without cutting the preform after polymerization).
In another embodiment, the invention involves an optical preform article. The preform includes a polymeric sheathing, which includes a sheathing polymer, that is at least partially transparent to at least one wavelength of light and has a refractive index of a first value at that wavelength. The preform also includes a polymeric core that is coaxially disposed within the sheathing and is at least partially transparent to the same wavelength(s) of light as the polymeric sheathing, and which has a refractive index of a second value at the central axis of the core at that wavelength that exceeds the first value of the sheathing. The polymeric core includes a core polymer and a core dopant having a refractive index which is greater than that of the core polymer. The core dopant is present in the polymeric core at a specified overall concentration. Furthermore, the preform is constructed and arranged to be formable into an optical fiber that conducts light at the above mentioned wavelength(s) with an attenuation of less than 500 dB/km, with the specified overall core dopant concentration not exceeding 7.9% wt.
In another aspect, the invention involves a plastic optical article. The article comprises a polymeric sheathing, which is at least partially transparent to at least one wavelength of light and a polymeric core, polymerized within the sheathing, which is also at least partially transparent to the same wavelength of light. The polymeric sheathing includes a sheathing polymer, and the polymeric core includes a core polymer and a core dopant that has a refractive index greater than that of the core polymer. The refractive index of the central axis of the polymeric core has a value at the wavelength of light mentioned above that exceeds the refractive index of the polymeric sheathing at the same wavelength by at least 0.01. Furthermore, the maximum service temperature of the article is at least 40 degrees C., preferably 45 degrees C., and more preferrably 50 degrees C.
In yet another aspect, the invention provides a method for making a gradient plastic optical fiber. The method involves first forming a polymeric preform rod comprising a polymeric sheathing and a polymeric core coaxially disposed within the sheathing that has a gradient in refractive index in a specified direction. The preform is then hot-drawn at a rate of at least 3 m/min, preferably at least 4 m/min, and more preferably, at least 5 m/min, into a fiber. The fiber thus produced conducts at least one wavelength of light with an attenuation less than 500 dB/km.